President Obama won re-election to the White House on Tuesday, defeating Republican challenger Mitt Romney after a long, expensive and hard-fought presidential campaign.

Obama won the critical battleground state of Ohio, pushing him over a majority of electoral votes and prompting widespread celebration among supporters of the president.

With some states yet to be decided, Obama had won 274 electoral votes, a majority.

Exit polls of voters leaving their voting places suggested a razor-close outcome in a deeply divided nation, with the incumbent holding a small advantage, 50%-48%. That finding was from a survey of more than 23,000 voters nationally.

Romney, the former governor of Massachusetts, held a lead in the popular vote after two-thirds of the states had been decided But his loss of the state of Ohio was considered a huge blow to his chances as it was a state he had campaigned in repeatedly and needed to achieve an electoral vote majority.

Another key state was Florida, where the two were running neck-and-neck, the Democrat had a tiny lead.

One of the smallest of the battleground states, New Hampshire, went for Obama. So did Nevada, a state that went for Obama in 2008. Obama made 10 trips to the state, beset with the nation's highest unemployment rate and one of the nation's highest home foreclosure rates.

The president also won Pennsylvania, a state Romney looked to late in the campaign for a potential upset. Romney won North Carolina, a state that Obama narrowly carried four years ago, and had a narrow lead in Virginia, where Obama also won in 2008.

Romney won at least 22 states, including much of the South, Plains and mountain West states.

On the West Coast, Obama won California and Washington.

Obama won two of Romney's home states -- Massachusetts, where the Republican won one election as governor, and Michigan, where Romney was born. Romney also has a home in New Hampshire, where he lost.

Obama won at least 16 states and the District of Columbia as he carried the northeast and parts of the Mid-Atlantic region, including New York. He won his home state of Illinois as well as Vice President Biden's Delaware.

The real race was for electoral votes awarded by winning individual states, with 270 votes needed to win.

Exit polls of voters in key states showed the economy was the top issue on voters' minds, and on that and other key issues the nation remains sharply divided. It was clear that Obama would not do as well as he did in 2008, when he won with a 7.3% margin of the popular vote.

The exit polls suggested Romney was winning among men by double digits. Obama was winning among women, who were a focus of much of the campaign, but by a smaller margin than four years ago.

The polling also suggested Romney had a narrow advantage among suburban voters, who were critical for Obama's 2008 election. The president retained a strong lead in cities.

The president appeared to have gained an edge among late-deciding voters, the exit polls suggested. Among those who decided who to vote for in the last few days, 49% voted for Obama, 46% for Romney. Among those who said Obama's response to Hurricane Sandy was important to their vote, Obama was favored by almost two-to-one.

Across battleground states where the outcome was in doubt, and where both candidates focused most of their energies and dollars, more than a million political commercials aired on TV stations during the long campaign.

Romney made final campaign dashes to Ohio and Pennsylvania on Election Day. Biden matched the late GOP campaigning with his own Ohio appearance, while Obama made calls from a campaign office in Chicago and relaxed with a game of basketball.

Obama awaited the results in Chicago. Romney was in Massachusetts.

After 17 months and more than $2 billion spent by each presidential candidate, it was up to the people who flooded churches, schools and auditoriums to cast their ballots. Campaigns on both sides did last-minute blitzes to boost turnout and get out their supporters, and lines were reported at voting places all over the nation.

There were glitches and confusion, but by late afternoon things seemed to be going smoothly in most places.

The struggling economy was on voters minds.

"Business is rough. Everybody wants someone to blame," said Frank Robles, 45, who employs 15 at his North Las Vegas shoe store. Yet Robles is supporting Obama, saying he's not responsible for the worst of the economic crisis: "People need to give him a chance."

But A.J. Jotipra, 69, a retired IBM worker says Obama has had his chance. Jotipra lost his Henderson, Nev., house to foreclosure last year. "The last four years, Obama has done nothing," Jotipra says.

In Ohio, a key battleground for both sides, the atmosphere at some polling places was informal and happy, despite the intense campaign and barrage of negative TV ads.

Retired nurse Nancy Manion, 74, of Dublin, Ohio, was excited to vote for Romney "to put God back in schools." But she also was thankful that campaign ads would end.

"Too many ads, too much slander," Manion says.

In Northern Virginia, Robert Adams, who has doctorates in business and psychology, said he voted for Romney and had four words to describe the campaign: "Too long. Too noisy." He said that "after awhile I just had the mute button on the television all the time."

In Dixville Notch, N.H., one of two tiny New Hampshire villages that get to cast the first votes of the presidential election, Obama and Romney tied with five votes each - something that has never happened before.

"I'm bewildered, that's the best way to describe my reaction," said voter Peter Johnson, adding he didn't think that Obama would get that many votes.

Besides long lines at polling places there are disputes over voter identification. Tuesday's vote is already being challenged in some locations.

Both campaigns have assembled legal teams to leap into action at any hint of voting irregularities, such as voting machine malfunctions, allegations of voter intimidation and challenges to the legitimacy of absentee and "provisional" votes.

On Sunday, a Florida judge extended early-voting hours in Orange County - a key swing region of a prized battleground state - after Democrats sued to provide more time for long lines of people trying to cast votes.

New York and New Jersey were still scrambling to resolve voting problems created by Sandy. New Jersey said it would expand online voting for those whose polling places have been disrupted, a move New York election officials rejected. New York has also had to relocate polling places, which could create voting challenges.

Voters were taking special election shuttles from storm-hit areas and voting by affidavit from any polling place they could reach after officials put emergency measures in place.

"It's important because it's our day," said Agim Coma, a 25-year-old construction worker who lost his apartment and car to the storm but was first in line to vote in one New Jersey town. "No matter what happens - hurricanes, tornadoes - it's our day to vote."

Ohio, Pennsylvania and Florida - all critical swing states for the presidential election - have faced pitched battles throughout the summer over voter-identification laws. As late as last week, Ohio election officials were issuing new rules for what ID is required and how the voter's identification should be certified.

In New London, Pa., a long line snaked out the door as hundreds came to vote.

Poll workers asked voters for identification but didn't require it. State legislators passed a voter-ID law earlier this year, but a state court blocked its implementation out of concerns it would disenfranchise legitimate voters who couldn't get identification.

If voters did not have identification, they were given a flier explaining the law "for coming elections."

In Jefferson, Wis., a steady stream of voters filed into City Hall. Mark McQuin, 39, a project manager, voted for Obama but said what many people were thinking: "I'm ready for it to be over."

NBC news and other news agencies are projecting President Obama wins four more years in the White House.

Ohio and its 18 electoral votes proved to be just as pivotal as the campaigns thought. Just after 11 pm, NBC called Ohio for Obama, giving him 274 electoral votes. Obama tweeting his supporters: "We're all in this together. That's how we campaigned."

While Romney still held a lead in the popular vote, Obama is on the track to win as many as 300 electoral votes, and perhaps more; it takes 270 to win a presidential election.

Votes are still being counted in Virginia and Florida.

USA Today reports that Romney is on his way to a convention center in Boston to delver his concession speech; Obama will follow him with remarks in Chicago.